Most metals are susceptible to corrosion, in particular atmospheric corrosion. Such corrosion will significantly affect the quality of such metals, as well as that of the products produced therefrom. Although this corrosion may sometimes be removed from the metal, such steps are costly and may further diminish the utility of the final product. In addition, when polymer coatings such as paints, adhesives, or rubbers are applied to the metal, corrosion of the base metal material may cause a loss of adhesion between the polymer coating and the base metal. A loss of adhesion between the polymer coating and the base metal can likewise lead to corrosion of the metal. Aluminum alloys are particularly susceptible to corrosion as the alloying elements used to improve the metal's mechanical properties (e.g., magnesium and zinc) will decrease corrosion resistance.
Prior art techniques for improving corrosion resistance of metal, particularly metal sheet, include passivating the surface by means of a heavy chromate treatment. Such treatment methods are undesirable, however, because the chromium is highly toxic, carcinogenic and environmentally undesirable. It is also known to employ a phosphate conversion coating in conjunction with a chromate rinse in order to improve paint adherence and provide corrosion protection. It is believed that the chromate rinse covers the pores in the phosphate coating, thereby improving the corrosion resistance and adhesion performance. Once again, however, it is highly desirable to eliminate the use of chromate altogether. Unfortunately, the phosphate conversion coating is generally not optimally effective without the chromate rinse.
Recently, various techniques for eliminating the use of chromate have been proposed. These include coating the metal with an inorganic silicate followed by treating the silicate coating with an organofunctional silane (U.S. Pat. No. 5,108,793). U.S. Pat. No. 5,292,549 teaches the rinsing of a metal sheet with a solution containing an organofunctional silane and a crosslinking agent in order to provide temporary corrosion protection. The crosslinking agent crosslinks the organofunctional silane to form a denser siloxane film. One significant drawback of the methods of this patent, however, is that the organofunctional silane will not bond well to the metal surface, and thus the coating of U.S. Pat. No. 5,292,549 may be easily rinsed off. Various other techniques for preventing the corrosion of metal sheets have also been proposed. Many of these proposed techniques, however, are ineffective, or require time-consuming, energy-inefficient, multi-step processes.
Thus, there is a need for a simple, low-cost technique for preventing corrosion of metals, particularly aluminum or aluminum alloys, as well as for treating a metal substrate prior to applying polymer coatings such as paints, adhesives, or rubbers.